While starter lead-acid batteries for automobiles are discharged or stored, oxygen gas and hydrogen gas are generated at positive and negative electrodes, respectively, in the principles of their operation. Among these batteries, valve regulated lead-acid batteries cause oxygen gas generated at the positive electrode to be absorbed in the negative electrode in order to suppress hydrogen gas generation at the negative electrode. However, even in such a valve regulated lead-acid battery, when a battery charger at the vehicle side is broken or used in a high-temperature atmosphere, an overcharge occurs, resulting in that oxygen gas and hydrogen gas are emitted through a control valve to outside the lead-acid battery. During the emission of the oxygen gas and hydrogen gas, a trace amount of a dilute sulfuric acid electrolyte in the battery is also emitted in the form of mist (acid mist) to outside the battery in some cases.
In a case where a lead-acid battery for vehicles as described above is installed in an engine compartment, oxygen gas and hydrogen gas emitted from the lead-acid battery into the engine compartment is relatively rapidly diffused into the air, and thus there is little risk of burning the hydrogen gas. The concentration of the acid mist is zero or at a level which is low enough to incur little risk of human and material damage, such as health damage on vehicle passengers and pedestrians when they inhale the acid mist and corrosion of the inside of the engine compartment.
On the other hand, so-called hybrid vehicles using engines and motors as their power sources have become widely available in recent years. In the hybrid vehicles, although Ni-MH batteries and Li secondary batteries are employed as the power sources of the motors, lead-acid batteries are still employed as batteries for auxiliaries of the vehicles.
Unlike ordinary vehicles, in a hybrid vehicle, a lead-acid battery for auxiliaries is placed in the cabin, such as under the sheet, or in the trunk. Such a difference in a place where the lead-acid battery is placed causes the necessities of capturing oxygen gas and hydrogen gas emitted from the lead-acid battery and a trace amount of acid mist, which presented no problems in the past, and of releasing these captured substances to outside the cabin.
For example, Patent Document 1 shows a structure which collectively emits gas generated in cell compartments, specifically a structure in which an exhaust vent is provided in the side surface of a lid of a battery to be connected to an end of an exhaust pipe and the other end of the exhaust pipe is guided from the cabin or the trunk to outside the cabin.
In the battery shown in Patent Document 1, a recess provided in a lid (corresponding to a cover) is covered with a cap (corresponding to a top lid) in order to collectively emit gas generated in the cell compartments, and thus it is necessary to assure hermeticity at a joint between the lid and the cap. When hermeticity is not assured at the joint, gas and acid mist leak from a gap between the cap and the recess, and thus these gas and acid mist accumulate in the cabin or trunk. This phenomenon is obviously undesirable in terms of safety or hygiene.
Lead-acid batteries for auxiliaries of hybrid vehicles are required to be small. For this purpose, as shown in Patent Document 1, ultrasonic welding is employed in joining the cover and the top lid to each other. This is because the top lid is made of a thin plate having a thickness of about 1 mm to about 2 mm in a small lead-acid battery, and thus heat welding using an ordinary heating plate might cause deformation of the entire top lid.